This invention pertains to a wafer bonding method and, more particularly, to a void free wafer bonding method.
In prior art, bonding of two silicon wafers is carried out at atmospheric pressure. Before bonding the wafers are treated with a solution to form hydrophilic surfaces. The wafers are then brought into contact, with polished sides facing each other, under extremely clean environmental conditions. While the wafers do bond together, it has been found that the bonding yields are relatively low since the two bonded wafers are plagued by the presence of areas of poor bond (voids) between them. Evidence indicates that the main reason for the formation of voids is the presence of air between the two bonding surfaces. The air trapped between the two surfaces causes not only a poor contact at room temperature but also a repulsing force between the two bonded wafers at high temperatures. The pressure of the repulsing force is estimated to be 75 psi when the bonding is carried out at 1100 degrees centigrade.